Personally I am positive it is an illusion - how he does it I have no idea but that doesn't mean it isn't a trick. I am sure I will be villified by the other ABers, however I simply do not believe it. i can't find any sites that discuss this altho there's quite a lot of "allegedly" and supposedly" out there.
I will echo dirtyegg, he looked like himself when I was there, and add that he is on sky 24/7 (though not on my set) and though they may be able to overwrite the video feed to cheat, someone watching would see the join. It would be pretty hard to conceal a switch, with the viewers there and TV someone would notice.
Personally, he seems so in his own world I believe he has the determination to do it as he seems to have lost much of his fear, and therefore is prepared to do it regardless.
Of course he is in the box, but my view is that he will have gone when the time comes for him to come out, I think he will turn up somewhere else, maybe America